Some current aftertreatment technologies utilize an SCR catalyst and injection of a reduction agent into the exhaust stream upstream of the SCR catalyst. On the catalyst, the NOx in the exhaust stream is reduced to N2 (nitrogen) and H2O (water). One common type of reduction agent is anhydrous ammonia (NH3), provided in a urea solution. When mixing with the exhaust gas beyond a certain temperature, the water portion of the urea solution is vaporized and the urea decomposes to form NH3 and carbon dioxide (CO2). The NH3 reacts with NO and NO2 over the SCR catalyst to form N2 and H2O.
The current methods of dosing urea solution into the exhaust gas under certain conditions can result in impingement of urea solution droplets on the walls of the exhaust tubing. While bulk exhaust gas temperatures are theoretically sufficiently high to decompose the urea solution to gaseous NH3, the impingement of droplets of urea solution results in localized cooling on the walls of the exhaust tubing. Where this localized cooling exists, the urea solution does not fully decompose which results in solid deposit build-up that causes restrictions in the exhaust gas flow, and reduction in NOx conversion over the SCR catalyst due to insufficient supply of NH3. Current strategies to mitigate this problem include avoiding injection of urea solution into the exhaust system until the exhaust gas temperature is sufficiently high to decompose the urea solution, and relying on manipulation of exhaust control strategies to increase exhaust temperature to remove solid urea deposits. However, this can result in additional NOx emissions, particularly during cold start and low duty cycle conditions, in addition to negatively impacting fuel economy. Even systems that currently preheat the urea solution prior to injection may have limited applicability in cold start and low duty cycle conditions due to the potential that in certain conditions exhaust gas temperatures are not sufficient to decompose the urea solution and prevent condensation or droplet formation on the walls of the exhaust tubing. Therefore, further improvements in this area of technology are needed.